1. Field of the Invention
The present invention relates to a lens driving device for moving a lens supporting member that supports a lens along an optical-axis direction.
A lens driving device according to the present invention can be used in an image pickup apparatus serving as an optical apparatus, such as a camcorder or a digital camera.
2. Description of the Related Art
Japanese Patent Laid-Open No. 5-27149 describes a lens driving device that includes a coil spring (hereinafter referred to as a rack spring) serving as one urging portion and having the function of reducing looseness of both engagement with a lead screw (feed screw) of a rack and attachment to a lens support.
Japanese Patent Laid-Open No. 8-248284 describes a lens driving device that includes a lens barrel having substantially the same fundamental configuration as in the above patent document.
However, the lens driving device has a pressing portion that faces a tooth-shaped portion (hereinafter referred to as a main rack tooth) engaging with the feed screw of the rack and that holds the feed screw. The pressing portion includes a projection having a triangular shape in cross section (hereinafter referred to as a pressing tooth).
Japanese Patent Laid-Open No. 6-174992 describes a lens driving device in which a pressing portion of a rack is arranged not in parallel to but at a predetermined angle to a facing main rack tooth to apply an urging force to a lens support in a particular direction.
The pressing portion of the rack in the traditional lens driving device described in any of the above patent documents has elasticity, and is inclined inward by the action of a load from the rack spring in a state before the feed screw is incorporated into the rack.
However, in the traditional lens driving device, the positional relationship between the main rack tooth and the pressing tooth does not match an optimal position for engagement with the feed screw in a state where incorporation of the feed screw into the rack has been completed.
Thus, in a state where the pressing tooth is inclined inward, when the feed screw starts engaging, a displacement is present in the positional relationship between the main rack tooth and the pressing tooth.
That is, when the main rack tooth and the pressing tooth start engaging with the feed screw in incorporating the feed screw from an opening side of the rack, a position displacement is caused in the pressing tooth by the inclination.
Therefore, the pressing tooth may enter a root of the feed screw adjacent to a correct root with which the pressing tooth should engage.
The base of the pressing portion including the pressing tooth is thin to minimize rigidity and have elasticity. Thus, when the pressing tooth is guided to such an incorrect adjacent root of the screw, a problem arises in which the pressing portion being warped incorrectly engages in the adjacent root.
FIGS. 10A to 10E illustrate, in a cross section of a lens driving device, a process for incorporating a feed screw 8a into a rack 18.
For the sake of illustration of a pressing portion 18g, a part of the rack 18 that is in front of the pressing portion 18g is not illustrated in the drawings.
FIGS. 9A and 9B illustrate a traditional rack. FIG. 9A illustrates the rack from an opening side; FIG. 9B is a perspective view thereof.
The rack 18 includes a pressing tooth 18b and a main rack tooth 18a. 
FIG. 10A illustrates a state before the feed screw 8a is incorporated. In this state, the pressing tooth 18b of the pressing portion 18g is inclined inward by a load of a rack spring 17a. 
FIG. 10E illustrates a state where the incorporation of the feed screw 8a has been completed.
The angle of the engagement portion between the main rack tooth and the pressing tooth illustrated in FIG. 10E in the state where the incorporation has been completed is represented by C. The angle of the engagement portion between the main rack tooth and the pressing tooth illustrated in FIG. 10C in a state where the main rack tooth and the pressing tooth start engaging in an incorporating process is represented by B.
The amount of displacement from a root of the feed screw 8a with which the pressing tooth should engage in FIG. 10C is (B−C)/360×screw lead L. When this amount of displacement is equal to or larger than one-half of the screw pitch, the pressing tooth tends to be guided into an adjacent root.
For example, when B is 255°, C is 150°, and L is 0.6 mm (double screw with a pitch of 0.3 mm), the amount of displacement is 0.175 mm. Because this amount of displacement is larger than 0.15 mm, which is one-half of the screw pitch 0.3 mm, the pressing tooth tends to enter a root of the feed screw 8a adjacent to a correct root with which the pressing tooth should engage.
In particular, when the lead of the feed screw 8a is large, the previously described displacement in a state where the pressing portion 18g is inclined is large, and this may lead to incorrect engagement.
Similarly, when the angle of the pressing portion (angle θ in FIG. 5) for applying an urging force to the lens support is large, as in the lens driving device described in the second patent document (Japanese Patent Laid-Open No. 8-248284), the above described displacement in the state where the pressing portion 18g is inclined is large, and this results in the occurrence of incorrect engagement.
When a driving unit i.e., a motor unit including the feed screw 8a is incorporated, an inner engagement state cannot be observed. Thus, incorrect engagement cannot be checked after the screw is incorporated, and the incorrect engagement may be present in a back-end process.
In a state where the pressing tooth is incorrectly engaged, the urging force of the rack to the feed screw is reduced, and this may lead to a malfunction caused by looseness of engagement.